1. Field of the Invention
The present invention relates to an amplifying device, and particularly to an amplifying device suitable for use as a recording amplifier which is connected to the primary side of a rotary transformer in a video recording device.
2. Description of the Related Art
Some video tape recorders (hereinafter referred to as "VTR") have recently employed an arrangement in which the diameter of a rotary drum, the contact angle of a magnetic tape on the rotary drum, and the rotational speed are respectively two-thirds, 1.5 times and 1.5 times the corresponding dimensions of a conventional two-head helical scanning VTR, as well as having a magnetic head for signals which comprises four heads shifted 90.degree. apart from each other. This arrangement allows miniaturization of the device to be realized while retaining compatibility with conventional magnetic tape formats.
There is not only a demand for miniaturization of such devices but also one for reductions in the level of power voltage needed. It is possible to have an optimal recording current flow through a recording head at a low voltage by causing a DC bias current to flow through the primary side (the recording amplifier side) of a rotary transformer at the stage before the recording head together with the recording current. In the case of recording with a VTR equipped with the four heads described above, even though three to four magnetic heads are always brought into contact with the magnetic tape, it is preferable to cause the recording current and the DC bias current to flow solely through a magnetic head which is actually taking part in the recording. The DC bias current is therefore also switched at the same time as the recording current.
FIG. 1 is a drawing of an example of the configuration of a conventional recording device. In FIG. 1, reference numerals 10a and 10b denote recording heads; reference numerals 12a and 12b, rotary transformers; reference numeral 14, a recording signal generating circuit for generating a recording current; reference numeral 16, a head switching signal generating circuit for generating a head switching signal; reference numeral 18, an analog switch; reference numeral 20, a current driving transistor, reference numeral 22, a resistor for providing a direct current bias for the transistor 20; and V.sub.B, a supply voltage. The recording current and the DC bias current flow through the primary side of the rotary transformer 12a only when the head switching signal output from the generating circuit 16 is at a high level. FIG. 2(a) shows the waveform of the current flowing through the primary side of the rotary transformer 12a, and FIG. 2(b) shows the head switching signal generated by the generating circuit 16.
In FIG. 2(a), the recording current is shown as if its amplitude were higher than the amplitude of the DC bias current. In point of fact, however, the amplitude of the DC bias current fed to the base of transistor 20 is higher than that of the recording current. Such fact is evident from a consideration of the function of the DC bias current. FIG. 2 is thus to be appreciated as not drawn to scale and so drawn in the interests of facilitating an understanding of the invention.
If, for example, the head 10a is being used without the head 10b being used, crosstalk is produced in the rotary transformer 12b in the unused channel from the rotary transformer 12a. At this time, the primary side of the rotary transformer corresponding to the unused head is short-circuited by a mechanical switch or an electronic switch such as a transistor or the like so that it is possible to prevent the frequency components contained in the recording current from being adversely affected by the crosstalk. During switching of the DC bias current, however, an induced current having a frequency spectrum with distribution up to a low frequency is caused to flow and this affects the other channel owing to the crosstalk and thus disturbs or erases the recording magnetization pattern when the unused head is in contact with the magnetic tape.